You can always go smaller on doors, though i imagine you'll run up against local building codes if you go too small. I want to say the interior doors at my house are 30'', but just check to see what your builders put in the other rooms (they should be to code already). Going much smaller than 30'' and you're in the "closet door" range and will have issues getting furniture in and out. Your door is always going to be the weak link so, other then furniture moves, there isn't a lot of benefit to huge doors. If i remember right, your rough-in should be roughly two inches taller/wider then your planned door.
If your hinges can go on the outside (bedroom side) of the door jamb, your life is easier. You can just order a wider jamb/casing (one made for a 2x6 frame or a custom one that's even wider) and install it into the existing rough opening, hinges going into the jack/king studs on the bedroom side. You'll probably have to add some plywood backing to the jamb on the theater side to close up the gap left between the 2x4 and the trim molding. Seal it all up with acoustic caulk.
If your hinges need to (or really want to) go on the theater side, it's harder, but doable. You'd have to redo your rough-in with wider boards (2x6 or wider, depending on how far into your room you want/need to go). I'd probably look at adding the boards to the existing jack studs, cantilever them out into the room a little, and install the drywall up to the edge of that new board. That solution could depend in your door and how heavy you expect it to be (normal door, solid core, solid core + sheets of mass and dampening, etc...). Heavier doors will need longer screws for the hinges, which needs thicker studs and maybe some bracing.
I'm not an expert on that one though, so someone with more carpentry experience can feel free to speak up.